There are two common types of industrial or municipal incinerators. One is a continuous feed system in which waste material is continuously fed to the combustion chamber and non-combustibles are continuously discharged, and the other is a starved air system which operates on batch principles.
The continuous feed incineration system includes a series of grates which supports the waste and air is passed upwardly through the grates into contact with the waste material, while the ash and non-combustible materials pass downwardly through the grates and are collected in a water filled trough. Waste is normally a low energy heat source, and as a result, certain materials such as glass, plastics, and lower melting point alloys, will melt and form a slag which is apt to clog the grates and prevent air from passing through the grates into the mass of waste material. To counteract this, the conventional continuous feed system normally will include a mechanism for agitating or moving the grates to dislodge the slag and permit air to pass upwardly into the waste material.
It has been found that in the conventional continuous feed system, up to 30% of the combustible material may pass through the grates along with the ash and non-combustible materials, thereby substantially reducing the efficiency of the system.
Because of the problem in introducing adequate air through the grates into the waste material the conventional continuous feed unit requires a relatively large volume combustion chamber in order to adequately burn the combustible waste gases. As the waste gases normally contain a substantial quantity of fly ash, the conventional procedure is to pass the waste gases through a scrubber where the gases pass through a tortuous path and are subjected to a water spray which acts to wash out the fly ash particles.
As a further disadvantage, the conventional continuous feed system will not adequately burn waste material that contains substantial quantities of water or moisture.
Furthermore, expensive and sophisticated controls are required with the process to monitor various conditions in the system. For example, controls are required to monitor the air pressure in the combustion chamber to determine whether the grates are partially clogged, and to monitor the temperature in the combustion chamber and thereby control the air input through the grates to obtain proper burning. Furthermore, temperature conditions in the scrubber are required to be monitored in order to control the amount of water added to the scrubber. Because of the complexity of the controls, the cost of the incinerator is substantial, so that in many cases other waste disposal systems, such as land fill systems, have been used rather than an incinerator.
The starved air type of incinerator is a batch system in which air is injected in minimum quantities and does not utilize mechanical grates. With the starved air system there is no agitation of the waste material within the combustion chamber, except for the introduction of new waste material. As a consequence, the starved air system is generally used only for dry materials, for dense compacted materials, or water laden materials, will not adequately be combusted due to the lack of agitation in the combustion chamber.
In the conventional starved air system, waste gases are normally carried off to a secondary combustion chamber, so that less fly ash is developed, as compared to a continuous feed system.
However, the starved air system has distinct drawbacks; the primary one being that the system is a batch type in which the ash and non-combustible materials remain in the combustion chamber and must be cleaned out of the combustion chamber after the burn out is complete. Moreover, the starved air system is generally a small volume unit and is limited to relatively dry material. The system does not adequately burn plastics, asphalt-based materials, or other wastes that will melt before they burn, as materials of this type will produce a solidified slag or mass in the combustion chamber.